Preparation And Investigation On The Performance Of QLED Using ZnO/Cs₂CO₃ As Electron Transport Layer

Quantum dot light-emitting diode?QLED?has become a strong competitor in the next generation of display and lighting fields because of its unique light-emitting characteristics such as high brightness,high color purity,and continuously adjustable emission spectrum.At present,most of the research on QLED is still in the laboratory stage because of the low efficiency and short lifetime of QLED device.The structure with hybrid organic-inorganic carrier transport layers is widely used in QLED device,which is composed of the anode,organic hole transport layer,light emitting layer,inorganic electron transport layer,and the cathode.The p-type organic semiconductors,such as PVK,Poly-TPD,TFB,etc.,are used as hole transport materials and n-type inorganic semiconductor ZnO is used as an electron transport material.While the hole mobility of these organic semiconductor materials is smaller than the electron mobility of ZnO,the injection and transportation efficiency of electrons in QLED device is higher than the hole,resulting in electron-hole injection imbalance and QLED device efficiency roll-off.Moreover,fluorescence quenching of the quantum dots may be caused by directly contacting between the quantum dot and the defect states in ZnO surface,which further limits the efficiency of the QLED device.In this dissertation,green QLED device was constructed using ZnO as the electron transport layer.We explored the mechanism that exciton quenching at the interface between QD/ZnO and charge injection imbalance lead to lower luminous intensity and efficiency of QLED device.The exciton quenching at the interface between the QD/ZnO and electron injection efficiency can be effectively reduced by the replacement of ZnO with ZnO/Cs₂CO₃ and Zn_1-xMg_xO/Cs₂CO₃ as the electron transport layer.The efficiency of QLED device was improved by adjusting the electron injection barrier which can reduce electron injection efficiency and promote effective radiative recombination of electrons-holes in the light-emitting layer.Since Cs₂CO₃ is a kind of stable inorganic material,the stability and lifetime of the QLED device can be improved by introducing Cs₂CO₃.The work can be summarized as follows:?1?Application of ZnO/Cs₂CO₃ as electron transport layer in green QLED device:The green QLED device with ZnO/Cs₂CO₃ as an electron transport layer was constructed by a solution method.By adjusting the solvent,concentration,rotating speed,annealing temperature of Cs₂CO₃,and concentration,rotational speed of ZnO layer,the optimal parameters of ZnO/Cs₂CO₃ as the electron transport layer were determined.Compared with standard devices,the luminous intensity,efficiency,and lifetime of green QLED device with ZnO/Cs₂CO₃ as the electron transport layer had been improved.It was found that the fluorescence lifetime of quantum dots was increased through the characterization of transient fluorescence spectra,and ZnO/Cs₂CO₃ as an electron transport layer suppressed exciton quenching at the interface between QD/ZnO.At the same time,the current density of electron only device with ZnO/Cs₂CO₃as an electron transport layer was reduced,which proved that ZnO/Cs₂CO₃ can effectively reduce the electron injection efficiency.The electron-hole injection in the QLED device tended to be balanced,so that the luminous efficiency can be improved.Compared with the standard device,the maximum brightness of green QLED device with ZnO/Cs₂CO₃ as the electron transport layer can be increased to 45150 cd/m²,the maximum EQE was 12.09%,and the lifetime reached 53288 h,with an increasement of 22%.?2?Application of Zn_1-xMg_xO/Cs₂CO₃ as electron transport layer in green QLED device:The Zn_1-x-x Mg_x O/Cs₂CO₃ was used for the electron transport layer of the green QLED device.By adjusting the doping concentration of Mg,the optimum doping concentration of 2%?x=0.02?is obtained,at the same time,the performance of the device was obviously improved.The electron injection barrier can be increased through the introduction of Zn_1-xMg_xO,so that the purpose of balancing the charge can be achieved.The performance parameters of green QLED devices with Zn_1-xMg_xO/Cs₂CO₃ as electron transport layer were further improved.The maximum external quantum efficiency was 13.85%.It is worth noting that the EQE was still kept at a high level when the brightness is in the range of 100-10000 cd/m²,and there is no phenomenon of efficiency roll-off,which paved the foundation for the commercial application of QLED in the field of lighting and display.